Typically, power combiners are used in RF transmitters to combine the output signals of parallel power amplifiers into one high power RF output signal for wireless transmission. In these known transmitter structures the signals are first amplified by the power amplifiers and then they are combined by a power combiner to produce a combined amplified signal for transmission. Depending upon the circuit architecture and signal format used, however, it becomes necessary to make trade-offs between reducing power losses and achieving isolation between input signals of the combiner.
The need for efficiency is a particularly important design factor for the highly integrated requirements of transceivers used for wireless local area networks (LANs) and employing modulation formats such as OFDM (Orthogonal Frequency Division Multiplex). Moreover, the assignee of this invention and application has developed signal modulation methods, using OFDM signal format, whereby information signals are deconstructed into independent component signals, these independent signals being more efficiently processed and modulated than the original information signals from which they derive, and then the independent signals are up-converted, amplified and combined prior to transmission. Use of such independent modulated signals presents additional challenges to achieving efficiency at the amplification/combination stages of the transmitter, however, because the conventional model of amplification followed by combining, using known power amplifiers and combiners, is subject to inherent loss and isolation limitations.
Non-reciprocal combiners are considered to be non-economic for applications such as low cost wireless. Instead, reciprocal combiners, realized as either four-port or three-port structures, are available for use in such applications. Four-port combiners provide an advantage of isolation between the individual inputs (which means that the output impedances of the amplifier stages do not load each other) but where the signals being amplified are non-identical (i.e. statistically independent) an inherent loss of 3 dB results (this loss disappears where the signals are identical due to resonance). Thus, four-port combiners are generally only suitable for use where the signals being amplified are identical.
A three-port combiner, also known as a trifilar, is able to provide a degree of isolation between its individual inputs, depending upon the output impedance of the amplifiers feeding it and the load impedance connected to the combiner's output. If the output impedances of the individual amplifiers and the output loading impedance of the combiner are the same, then isolation is not achieved and an inherent loss of 3 dB results. On the other hand, if the output impedances of the amplifiers are small in comparison with the output loading impedance of the combiner, then the inherent loss diminishes, and approaches 0 dB for 0 ohms output impedance.
The many classes of power amplifiers can be broadly sorted into two classifications; linear and switched-mode. Linear amplifiers provide an output-impedance resulting from the bias condition and load line for the active device (in the usual case, the active device being a transistor). In practice, this output impedance is typically in the range of 5 to 50 ohms. As a result, limited isolation is achievable when using a three-port combiner (trifilars) to combine the outputs of two linear amplifiers. As known by persons skilled in the art, a conventional class D, E or F switched-mode power amplifier consists of an input component having at least one active (switching) device, a central transformer component and an output component consisting of a resonator. It is impractical to apply the output signals of separate switched-mode amplifiers to a trifilar, to combine them, because of the cost and space requirements (and resulting inefficiency) associated with the multiple transformer windings required for such a design.
By reason of the foregoing limitations of known RF components, there exists a need for new and efficient means to achieve power amplification and combining of modulated signals in transmitters.